Regarding nitrogen fixation in leguminous trees, the conventional wisdom has been to divide the family into nodule-forming and non-nodulating species, and to assume that those species without nodules do not fix nitrogen.
This conventional wisdom has now been challenged by Jim Bryan in his 1995 doctoral dissertation for the Department of Forestry at Yale University. Bryan has found evidence that non-nodulating leguminous species apparently do form a symbiotic relation with soil bacteria (Bradyrhizobium spp. or Rhizobium spp.). This endosymbiosis is carried out directly in the roots of the trees rather than in nodules. Honeylocust (Gleditsia triacanthos L.) was the primary species used in Bryan's investigation.
To establish the presence and activity of rhizobial bacteria in the roots of honeylocust, Bryan utilized three techniques: light microscope with stained root slides, scanning electron microscope (SEM), and acetylene reduction. Each technique indicated the possible presence or activity of the rhizobial bacteria inside the honeylocust root cells. The electron micrographs of the rhizobial bacteria in honeylocust roots are impressive. Using the SEM, Bryan also found evidence of bacterial symbiosis in other non-nodulating legume species.
Bryan emphasizes that his research strongly supports, but does not prove, the non-nodule nitrogen fixation. He is continuing his investigation with acetylene reduction of honeylocust seedlings grown under more exacting conditions (in sand and hydroponically), and is also observing other non-nodulating legumes with the SEM.
Interestingly, Bryan initiated his study from an agroforestry perspective. He began by cataloging members of Leguminosae that produce beans (pods) for human consumption. During this cataloging he found that a disproportionate share of the bean producers did not present nitrogen-fixing nodules. He then set out to develop a practical test to identify nitrogen fixing bean producers, and this led him to discover the non-nodule rhizobial symbiosis.
In the approximately 18,000 member Leguminosae family, Bryan found 277 species which produced beans for human consumption. Of the 132 species in this group presently tested, 30% did not present nitrogen fixing nodules, compared with only 11% for the family as a whole. Of the 277 bean producing species, only 9 are native to the temperate zone.
The Gleditsia genus evolved comparatively early within Leguminosae. The nitrogen fixing in honeylocust is in several aspects 'primitive' compared to that found in nodulating legumes. This suggests to Bryan a possible evolutionary continuum within Leguminosae from the non-nodulating species of the Caesalpiniodeae subfamily to the nodulating species.
The implications of Bryan's findings for honeylocust agroforestry are unclear. How much free nitrogen honeylocust will produce, and how much of that nitrogen will be available for grass production is unknown. Almost certainly there will be increased interest in honeylocust. Whether this interest will be justified on the basis of additional free nitrogen will require considerable research.
Bryan, James A. 1995. Leguminous Trees with Edible Beans, with Indications of a Rhizobial Symbiosis in Non-Nodulating Legumes. Doctoral Dissertation, Yale University.
Copies of Jim Bryan's thesis can be obtained from:
University Microfilm International
P.O. Box 1764
Ann Arbor, MI 48106 USA
Telephone: (800) 521-3042
Cj Sloane wrote:http://www.savannainstitute.org/honeylocust.html